Cassava

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Cassava Cross Section


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General Information

Cassava (Manihot esculenta) is a commonly produced tuber crop in Africa. It can be used as food, as a cash crop, as feed for animals and as a source of industrial raw material. In sub-Saharan Africa, cassava is mostly used for human consumption in various forms ranging from boiling the fresh tuber to processing it into cassava flour. Cassava tubers are an important source of carbohydrates, while the leaves, eaten as a vegetable, are a good source of protein and vitamins.

Challenges facing cassava production in Africa

  • Low productivity. Although cassava is an important crop with multiple uses, it does not receive the much needed attention during its production. Farmers normally plant it on very poor soils, where other crops like maize have failed. Sometimes cassava is grown as an insurance intercrop with other nutrient-demanding crops like maize or sorghum, just in case the main crop fails. Cassava is predominantly a crop for small-holder farmers, who basically grow for subsistence, using rudimentary tools and operating on small and fragmented plots. Crop yields are further reduced by infections by the cassava leaf mosaic disease, the cassava brown streak disease and the cassava mealy bugs and scales.
  • High postharvest losses. Poor postharvest handling leads to uneven quality of the processed cassava and results in contamination by fungi. Poor and inadequate facilities for milling and storage; and poor access to roads, which are vital for adding value, further increase the postharvest handling challenges.

Cassava remains easy to produce, adaptable to many environments, with minimal labour requirements and less susceptible to pests and diseases. However, there is need to address increased productivity, marketing opportunities and profitability of cassava production. The following organic practices can contribute to achieving these goals.

Establishment of the cassava garden

In organic farming, crop management begins by giving the plants good growing conditions through improving soil fertility, and healthy planting material. This allows the crop to grow healthier, and produce higher yields.

Suitable varieties for organic production

Cassava varieties differ with regard to yield potential, flesh colour (white or yellow-fleshed), diameter and length of the tubers, disease and pest resistance levels, time from planting to harvest, cooking quality and taste. Some cultivars require 18 months or more from planting to harvest, while others are ready to harvest in 9 months. Most cultivars have been selected by farmers under their growing conditions based on yields and cultural tendencies. Each growing region has its own special cultivars with farmers, often growing several different cultivars in the field at the same time.

Recommendations to farmers for selecting suitable cultivars

The best cassava varieties are those preferred by consumers. They grow fast, give good yields, store well in the soil and are tolerant to major pests and diseases. The following criteria are useful for selecting cassava varieties for organic production:

  • Good adaptation to local conditions. To adopt the right variety it is important to be aware of the general growing conditions for cassava including length of the rainy season, prevalent diseases, pests and weeds. Such information helps to determine what characteristics are needed by each variety to perform well.
  • Varieties with high dry matter and good food quality. Cassava varieties with tubers with dry matter content of more than 30% are said to produce good quality products and are more profitable for processing.
  • Adaptability to different uses. The selected cassava variety should be adapted to multiple uses such as food, animal feed or industrial processing. Varieties that are commonly preferred are with tubers that are tasty for home consumption, can store well for processing purposes and produce enough foliage for animals.
  • Ability to bulk early. Varieties that show early bulking, meaning the swelling of the root tubers, are better able to compete with weeds, than late maturing varieties, and are suitable for drier areas with short rains.
  • Ability to store well in the ground. Varieties that keep the tubers in good condition for a long time after reaching maturity are preferred. Good ground storability leaves more time for harvesting, thus reducing the duration of postharvest storage problems of fresh roots.
  • Resistance to local weeds, pests and diseases. Varieties that can tolerate the prevalent diseases and pests in the area are most preferable.

Selection of an appropriate planting site

Cassava is drought tolerant, can grow on most soils, and gives some yields even on poor soils where most other crops fail. However, high yields are obtained in areas with well-drained, loamy soils, well-distributed annual rainfall of 1,000 to 1,500mm, and warm and moist climatic conditions only. The best site for planting cassava is flat or gently sloping land. Steep slopes are susceptible to erosion and are, therefore, not very suitable areas for growing cassava. Valleys and depression areas are also not recommended because they are prone to water logging. Cassava is sensitive to water logging and heavy soils do not allow the crop’s roots to proliferate and develop.

Land and seedbed preparation

In cassava cultivation, it is important to till the land to loosen up the soil, improve soil drainage and make it easy for roots to develop. The level of tillage required for the cassava field mainly depends on the soil type and the drainage at the selected site. In places with shallow soils or poorly drained clayey soils, it is important to make mounds or ridges onto which the cassava is planted, as it encourages better root development and yields. In sandy soils, only minimum tillage is necessary and the cassava can be planted flat into the soil, as the soil is sufficiently loose to allow root development.

Preparing good quality planting material

Cassava is propagated by planting pieces of the stem (stem cuttings). The development of cassava and amount of yields depends on the quality of stem cuttings. There are several cassava pests and diseases, which are stem-borne. Selecting healthy stem cuttings reduces the spread and damage caused by pests and diseases.

Recommendations to farmers in selecting good cassava stem cuttings

  • Select planting material from healthy growing, high-yielding, 8 and 18-month old cassava plants. Healthy cassava plants have robust stems and branches, lush foliage, and minimal stem and leaf damage caused by pests and diseases.
  • From each plant, select the middle, brown-skinned portion of stems as stem cuttings. The stems should be 2 to 4cm thick. These parts sprout and ensure plant vigour better than the top green stem portions. Stem cuttings taken from the top green portions or extreme top and bottom portions of stems are unsuitable. They will dehydrate quickly, produce unhealthy sprouts, and are easily damaged by pests and diseases.
  • Tie the stems in bundles and wait for at least 10 days before planting them. The harvested stems can be stored for over 2 months in dry, well-ventilated, shaded areas away from direct sunlight until it is time for planting. One simple method of storing stems consists of arranging them vertically under a shady tree, with the oldest part of the stem buried in the soil. The soil should be moist to keep the stems ‘alive’ as leaves will form on the upper part of the stems. After storage, discard the top and basal parts of the stems, and use the middle part as cuttings. Another method, mainly used under cold conditions, consists of storing the stems in underground tunnels, which are protected from water. The stems are placed inside the tunnel on top of a layer of dry straw, and then covered with another layer of straw and soil.

Planting

To get the best sprouting and growth from cassava stem cuttings, the following considerations are recommended:

  • Selection of suitable planting dates. Planting cassava early, at the beginning of the rainy season, ensures healthy sprouting and good plant establishment. This enables the plant to withstand attack by diseases and pests later in the season.
  • Preparation and handling of stem cuttings. When cutting up cassava stems into pieces for planting, each cutting should be between 20 and 30cm long and have about 5 to 8 nodes, where roots and shoots originate. The interval between cutting of the stems and planting into the ground should be as short as possible to avoid dehydration and poor performance. Soaking the stem cuttings before planting in warm water (50°C) by mixing equal volumes of boiling and cold water for 10 minutes just before planting prevents stem-borne pest attacks.
  • Adopt suitable planting mode according to the type of soil. Cassava cuttings can be planted by hand vertically, at an angle (inclined) or horizontally, depending on soil types. The drier the soil, the bigger the part of stem placed in the soil. The vertical planting method is best suitable in sandy soils and consists of planting the cuttings vertically with two-thirds of the length of the cutting below the soil. Planting at an angle is most suitable in loamy soils and consists of planting the stem cuttings vertically and with an angle ranging from slightly above horizontal to about 45°. Horizontal planting is recommended for dry climates and consists of placing the entire stem cutting horizontally in the soil at a depth of about 5 to 10cm. The spacing between the cassava plants depends on several factors such as the variety used, the soil type, soil fertility and water availability and on whether cassava is grown alone (mono crop) or with other crops (intercrop). If cassava is grown alone, a distance of 1mt between the plants should be considered. If cassava is grown as an intercrop, the distance between the crops should range from 1 to 4mt depending on the branching habit of both the cassava and other crops to make sure there is enough space for the plants.

Intercropping

Due to the fact that cassava has a slow initial development, intercropping during early crop development is feasible, and helps reduce soil erosion. However, farmers should consider that cassava is a poor competitor and can easily be shaded out by tall intercrops like maize. For this reason, it is important to consider the branching habit of both the cassava and the other crops in the intercropping system and make sure there is enough space for both crops. Furthermore, cassava can suffer from nutrient and/or water competition from intercrops. Therefore, attention must be given to the intercropping species that have different root systems and nutrient requirements.

Farmers usually intercrop cassava in simple or complex mixed cropping systems with vegetables such as amaranth and okra, plantation crops such as coconut, coffee, maize or legumes, and pulses such as cowpea and groundnuts. The intercropping pattern depends on the environmental conditions, food preferences and market conditions of the region.

Simple mixtures consist of the intercropping of only two crops, in which farmers select arable crops on the basis of differences in growth habit and time of maturity. For example, cassava, which is a long-duration crop with 9 to 18 months to maturity, is often intercropped with short-duration crops with 2 to 5 months of maturity process, such as maize, cowpeas, groundnut, okra and melon. These crops mature when the cassava is just attaining its maximum leaf area development and thus is able to expand its root tubers without competition. In complex mixtures consisting of three or four crops, good yields have been obtained with the following combinations:

  • Maize - Cassava - Melon
  • Maize - Groundnuts - Cassava
  • Maize - Cassava - Okra - Cowpea
  • Maize - Yam - Cassava
  • Maize - Beans - Cassava

Complex mixtures improve weed suppression, reduce soil temperature, retain soil moisture in the topsoil, and produce more organic matter than single cropping or simple mixtures. Nutrient loss from erosion in complex mixtures is less than in single cropping.

Crop rotation

The continuous planting of cassava in the same field year after year leads to increased disease and pest levels, reduced yields and crop failure. To avoid such development, organic farmers should wait for at least 2 years before planting cassava on the same field again and develop a crop rotation system. A rotation system generally improves soil fertility, reduces soil erosion and helps to control diseases and pests. The suitable crop rotation depends on several factors such as the climatic conditions, the market requirements and the skills and objectives of the farmer. However, within a pattern of crop rotation, cassava is often grown in sub-Saharan Africa at the end of the sequence, as it can still produce relatively well at lower fertility levels, where other crops would not grow well.

This practice leads to lower cassava yields. It is important to establish a balanced crop rotation, which maintains or improves soil fertility, and to give cassava a place in the rotation that corresponds to farmer expectations. Cassava is a good crop to follow such crops as pumpkin, squashes, maize, sorghum or improved fallow. A 3-season rotation example that can be used in organic cassava production is maize-beans / cassava / groundnuts.

Reducing postharvest losses

Post-harvest handling of organic cassava aims at maximizing tuber quality by minimizing any damage or cuts on the tubers during harvesting and transportation of the tubers. Young leaves and shoots of cassava are also harvested to be consumed as vegetables and may be as important as tubers for generating income. However, excessive harvesting of the leaves can have a negative effect on the yield of tubers.

Timely harvesting

Early-maturing cassava varieties are ready for harvesting at 7 months, while late-maturing varieties are ready 12 months after planting. The proper stage for harvesting is when the leaves turn yellow and fall down and the roots are mature. It is advisable to harvest cassava once it is mature. If the tubers are left in the ground over long periods, they lose quality and become woody due to hydrolysis of starch to sugars. Care should be taken to avoid damage to the tubers during harvesting. Damaged roots are highly susceptible to fungal attacks and decay.

Harvesting cassava tubers is labour-intensive and done by hand. It is easy if the soil is sandy or during the rainy season, but in heavier soils or during the dry season, harvesting usually requires digging around the tubers to free them of the covering soil and then lifting/pulling the plant. The day before harvest, the plants are normally ‘topped’; the stalks are cut off 40 to 60cm above ground and piled at the side of the field. From this material, the stalks for the next planting are selected. Excess soil is then scraped off from the tubers by hand. This should be done carefully so as not to peel or damage the outer protective skin of the tubers.

Transportation

The first thing to be done after the harvest is to transport the tubers from the production and harvest field to the processing and utilization site. This is because fresh cassava is highly perishable (within 2 to 3 days after harvesting). Transportation of cassava tubers should also be done carefully to avoid bruising and dehydrating the cassava tubers, especially if it is meant for fresh consumption.

Preservation

Since cassava roots can remain in the soil for up to 18 months after reaching maturity, the simplest preservation technique is to delay the harvest until the crop is needed. However, this method has the following disadvantages:

Cassava roots increasingly lose starch, the constituent defining its value. They become fibrous and woody with prolonged in-ground storage. The longer the roots remain in the ground, the longer they become exposed to insect, disease, or rodent attacks. The land may be needed to plant other crops.

Freshly harvested tubers can be preserved by the following methods:

  • Cassava buried in straw-lined trenches and protected from seepage of ground water can hold for periods of up to 12 months. A shade is needed around the trenches; therefore, it is better to put several trenches under the same shade (roof).
  • Storage in tightly woven bags such as rice or cocoa sacks. With this technique, storage times of 7 to 10 days are achievable.
  • In a clamp storage system, a conical pile of 300 to 500kg of fresh cassava roots is seated on a circular bed of straw and covered with more straw. The whole unit is covered with soil to a thickness of 10 to 15cm, the soil being dug from around the clamp so as to form a drainage ditch. With this storage system, minimal losses up to 20% may be expected for periods of up to 2 months.
  • Storage of tubers in wooden crates containing absorbent material such as damp sawdust. However, if the sawdust is too moist, it may promote fungal growth and if it is too dry, the roots deteriorate quickly. Lining the crates with perforated plastic prevents dehydration of the sawdust, resulting in a storage period of about 1 to 2 months.
  • Cold storage by keeping the cassava tubers below 4°C. This system greatly reduces cassava deterioration and may be practicable for high-value markets. Alternatively, roots, or more commonly pieces of root can be stored frozen. This is a satisfactory method to conserve the tubers. It must be noted that, although the flavour is preserved, freezing changes the structure of cassava tubers, making them spongier.

Marketing and organic certification

Much of the cassava production is used at household food consumption level. It is also increasingly becoming a raw material for industrial production, especially for starch production. Organic certification of cassava production is only reasonable as a market requirement if there is a market that demands it. In such a case, interested farmers should be willing to adopt general organic production requirements, like not using synthetic pesticides and fertilisers, and applying other sustainable production methods.

Other considerations include:

  • Farmers should have enough land to produce cassava beyond the household requirement (commercial volumes), to be able to cover the extra costs of certification.
  • For successful marketing, farmers may need to work as group to increase production volumes needed to justify the cost of certification.

Specific national or international organic standards may define additional requirements for production and postharvest handling of cassava. Farmers should consult the national organic movement or organic certification body operating within the region or country.

Sources and References

Bibliography

Baier, A., (2005). Organic Certification Process. National Sustainable Agriculture Information Service. [Accessed 21 March 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Soil Fertility Management. [Accessed 19 March 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Conversion to Organic Farming. [Accessed 19 March 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Crop Management. [Accessed 21 March 2012]

FiBL (2011): African Organic Agriculture Training Manual. Version 1.0 June 2011. Edited by Gilles Weidmann and Lukas Kilcher. Research Institute of Organic Agriculture FiBL, Frick. [Accessed 12 July 2012]

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